Substance use/abuse (SU/SUD) and other HIV/STI risk behaviors are major public health challenges not equally distributed across racial/ethnic groups. The fastest growing minority population in the US, Latino youth, have elevated rates HIV/STI risk behaviors and SU/SUD, particularly among those abandoning traditional Latino values and shifting towards American values (acculturation). This application, in response to PA-09-236: HIV/AIDS, Drug Use, and Vulnerable Populations in the US (R01), aims to identify factors related to the development of SU/SUD and HIV/STI risk behaviors and infection among Latino youth during a critical transitional developmental period: emerging adulthood. Existing studies have revealed important disparities, however, they are limited in their ability to identify processes relevant for Latino youth, as they have not included sufficient numbers of Latino youth from fairly homogeneous subgroups (e.g. Puerto Ricans, Mexican- Americans, etc.) and lack information about acculturation, a key determinant of risk among Latino youth. We propose to conduct the fourth wave on the Boricua Youth Study (BYS~ R01 MH56401) focused on HIV/STI risk, infection in emerging adulthood (BYS Wave 4, N=1,992, youth ages 16 - 24). The BYS has already generated 3 waves of data (years 2001 to 2004) on children of Puerto Rican background (ages 5 -13 at Wave 1), with study findings described in 19 peer-reviewed publications to date. Besides the comprehensive longitudinal assessment strategy, a unique feature of the BYS is its two-site design, as it included children living in the South Bronx (SBx), NYC and San Juan, Puerto Rico (PR). At BYS Wave 4, we propose to conduct intensive assessment of HIV/STI risk (including testing for HIV and STIs) and enhance the assessment of SU/SUD, in addition to continuing tracking of cultural experiences, and psychiatric disorders and other factors. The proposed study has as its specific aims, to examine: (I) Prevalence of HIV/STI risk behaviors and rates of HIV/STI infection in emerging adulthood and the influence of sexual orientation, teenage unintended pregnancy and early sexual debut. (II) How development of SU/SUD (nicotine, alcohol, illicit/prescription drugs) starting in childhood/earlyadolescence may influence HIV/STI risk in emerging adulthood in the context of a Ecodevelopmental framework. (III) The role of acculturation (cultural practices, identification, values) in relation to SU/SUD and its relationship to HIV/STI risk behaviors/infection. Once Wave 4 is completed, the BYS will represent an unparalleled source of prospective information about onset of SU/SUD and HIV/STI risk behaviors and infection as well as persistence and progression of SU from childhood to emerging adulthood in a homogeneous Latino subgroup examined in two contexts (SBx and PR).This information will fill a gap in knowledge, and the model resulting from our investigation will enable and facilitate the integration of specific factrs of relevance for Latino youth (e.g., acculturation) as part of interventions for this high-risk group.